1. Field of the Invention
The present invention relates to a device and to an automatic method for the geometrical calibration of an X-ray imaging system. Systems of this type are used chiefly in the medical field in order to analyze internal structures of the human body, notably vascular structures. The aim of the invention is to enable the use of the system, by means of its easy calibration, for the quantitative assessment and, qualitative assessment of the dimensions of the internal structres analyzed. The invention relates essentially to systems using 2D radiation detectors, for example substantially plane radiation detectors. The invention also pertains to a method of rotating systems of X-ray imaging.
2. Description of the Prior Art
In an X-ray imaging system formed by an X-ray source and a 2D detector, whether rotating or not, the geometrical operation that plays a role in the production of the image is a conical projection of an analyzed 3D object on a 2D space which is that of the plane of projection corresponding to the plane of detection. The geometrical parameters that completely describe the conical projection are nine in number. These nine parameters may be presented according to different types of parametrization.
The following is one of the possible types of parametrization:
three first parameters relate to the three coordinates of the X-ray source S in the referential system R of the 3D space in which the analyzed object is placed; PA1 three other parameters relate to the three Euler's angles associated with the change of referential system between the referential system R of the 3D space of the object and a referential system R' related to the conical projection, i.e. a referential system formed by a 2D referential system of the plane of projection P and a third axis orthogonal to the plane of projection; PA1 three last parameters relate to the three coordinates of an arbitrary point I of the plane of projection P (for example the center of the image in the referential system R). PA1 a known object is available, namely a calibration phantom having a certain number of characteristic points whose position in space is known by coordinates measured with respect to a referential system proper to this object; PA1 the image of this phantom is acquired under the geometrical conditions of a viewpoint (or incidence) that is to be calibrated; PA1 the projections of the characteristic points in the image are recognized. For this purpose, each characteristic point of the object is associated with its trace in the acquired image, namely the projection; PA1 the system of equations describing the projection is inverted in the mathematical sense; PA1 and, finally, all of the parameters of the projection for the given viewpoint are obtained. PA1 (1) D. L. Parker, J. Wu, D. L. Pope, R. Van Bree, G. R. Caputo and H. W. Marshall, "Three-Dimensional Reconstruction And Flow Measurements of Coronary Arteries Using Multiview Digital Angiography", in J. C. Reiber and P. W. Serruys ed. New Developments in Quantititative Coronary Angiography, Kluwer Academic Publishers, 1988, pp. 225-247; PA1 a phantom of known dimensions is placed between the tube and the detector; PA1 a measurement is made, for a rotational position of the tube/detector assembly with respect to the phantom, and in the radiological image of the phantom projected on the 2D detector, of the coordinates of image loci of characteristic points of the phantom; PA1 a deducing is done, for this rotational position of the tube/detector assembly, and in a referential system associated with the position of the phantom, of the calibration coefficients pertaining to the respective positions of a focal spot of radiation from the X-ray tube and of the 2D detector; PA1 and these last two steps are reiterated for different desired positions of the tube/detector assembly, PA1 the calibration coefficients relating to a certain number of rotational positions of the tube/detector assembly are processed for the extraction therefrom of the intrinsic parameters of the X-ray imaging system which are independent of the rotational position of the tube/detector assembly; PA1 and these intrinsic parameters are deduced from the calibration coefficients pertaining to any rotational position of the tube/detector assembly by assessing the values of analytic functions of these intrinsic parameters for an angular value of rotational position of the tube/detector assembly.
To simplify the computations in the invention, it will be seen here below that there are reasons to prefer cylindrical coordinates for the first three and the last three coordinates. The referential system R is then a cylindrical referential system.
Knowledge of all or a part of these parameters is very often useful in radiology, notably when a quantitative data element pertaining to a 3D object is estimated from a measurement made in a 2D projection or in several 2D projections of this object. Parameters of each viewpoint must then be known. A viewpoint pertains to the orientation of the system with respect to the object. For example, we may take the standard problem of the estimation of the section of a vessel from one or more 2D projections. Now, gaining access to these parameters directly, i.e. for example by making direct measurement, on the acquisition system, of the distance between the X-ray source and the detector, is impossible or implies an excessive degree of imprecision.
The term "geometrical calibration of an imaging system" denotes the operation that leads to precise indirect knowledge of the geometrical parameters that play a role in the production of an image. The principle, which is a classic one, is based on the use of a geometrical phantom that is known in the 3D space and whose 2D projection is acquired. The sequence of the operations carried out to this end comprises the following steps:
In the prior art, the recognition of the characteristic points in the projection of the calibration phantom is done "by hand" by a human operator. For example, a frequently used form of geometrical calibration phantom is that of a cube, at the eight corners of which metal beads or bullets, opaque to light, are positioned. Sometimes, supplementary beads are added on to increase the geometrical precision of the calibration. Now, projection along the direction of the X-rays produces a "transparent" image of the object wherein, depending on the orientation of the tube, it may be very difficult to associate the 2D traces of the beads with their corresponding 3D points without making mistakes. The following articles may be referred to for the state of the art as regards geometrical calibration:
D. J. Hawkes, A. C. F. Colchester and C. R. Mol, "The Accurate 3D Reconstruction of the Geometric Configuration of the Vascular Trees from X-Ray Recordings" in R. Guzzardi ed. Physics and Engineering of Medical Imaging, Nijhoff, 1987;
M. Garreau, J-L Coatrieux, R. Collorec and C. Chardenon, "A Knowledge-Based Approach For 3D Reconstruction And Labelling Of Vascular Networks From Biplane Angiographic Projections" in IEEE Medical Imaging, Vol. 10, No. 2, June 1991, pp. 122-131.
This need for human intervention is a major drawback and may even become prohibitive in certain cases, when the number of viewpoints to be calibrated is large. This is especially so with any system for the acquisition of 2D projections by X-rays with a view to 3D reconstruction. A system of reconstruction such as this is described, for example, in the French patent application No. 2 644 590 filed on 20th Mar. 1989. Whatever the method of 3D reconstruction, it is necessary, first of all, to have perfect knowledge of the geometrical parameters that characterize each of the projections. In the probable case where an indirect calibration method is necessary, it would seem to be indispensable to carry out this calibration automatically.
The present invention therefore proposes a device enabling an automatic calibration that can be applied to any system of 2D X-ray imaging and in particular to any system used for purposes of 3D reconstruction.
The device of the invention is such that the unequivocal recognition of correspondences between the beads and their traces is automatic. This is obtained by choosing a phantom in which the beads are distributed, step by step or by degrees, in a sequence such that altitudes of beads, measured along the rotational axis of the imaging system and, especially, an axis of the phantom, are monotonically increasing (or decreasing) with an order number of the beads in the sequence.